Caspian Tern Colony Counts

East Sand Island

The high and low on-colony counts for the week on East Sand Island were 15,965 terns (May 9) and 11,335 terns (May 8), respectively. The number of terns counted on colony continues to increase and will likely reach a peak in 1-2 weeks. Most of the terns on the East Sand Island colony are incubating eggs.

Rice Island and Other Upper Estuary Islands

During this past week, no terns were observed roosting or attempting to nest in upland areas on Rice Island, Miller Sands Spit, Pillar Rock Sands, Fitzpatrick Island, Grassy Island, Crimms Island, and Browns Island (referred to as the upstream end of Puget Island in previous reports). No tern eggs have been found at any of these sites so far this season. The continuous hazing of terns prospecting at Pillar Rock Sands by USACE contractors was discontinued on May 11; hazing at Miller Sands Spit by USACE contractors will end on May 14. We will continue to monitor Pillar Rock Sands, Miller Sands Spit, Rice Island, and other upper estuary dredged material disposal islands 3 times each week looking for early signs of nesting (e.g., scrape digging) and will promptly inform resource managers of any such activity.

Washington Coast (Willapa Bay, Grays Harbor)

During aerial surveys conducted along the southern Washington coast this past week (May 11), no terns were observed roosting or attempting to nest on any of the islands in Willapa Bay or Grays Harbor. We will continue to fly periodic surveys in these areas looking for nesting terns.

Up-river Sites (Crescent Island, Three Mile Canyon Island, Miller Rocks)

The high and low on-colony counts for the week on Crescent Island were 800 terns (May 8) and 740 terns (May 6), respectively. No terns have been observed at the former colony on Three Mile Canyon Island so far this season; mink predation caused complete colony failure in 2000 and 2001 (no terns attempted to nest there in 2002).

This past week, surveys of gull colonies located on the lower Columbia River at Richland Island and Island 18 (both near Richland, Washington) revealed no nesting Caspian terns.

Diet Composition

For the week of May 5 – May 11, 67% of the identifiable fish delivered to the East Sand Island tern colony (N = 362) were salmonids, a nearly two-fold increase over the percentage for the previous week (36%). Other major prey items being delivered to the East Sand Island colony included smelt (17%) and clupeids (e.g., herring; 10%). Year to date, 33% of the identifiable fish delivered to the East Sand Island tern colony (N = 1,618) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 39% juvenile salmonids in the diet of terns on East Sand Island by the same date in 2002.

For the week, 83% of the identifiable fish delivered to the Crescent Island tern colony (N = 160) were salmonids, nearly the same as the percentage for the previous week (86%). Most of the non-salmonids observed being delivered to the Crescent Island colony were cyprinids (e.g., northern pikeminnow, peamouth; 16% of total diet). Year to date, 71% of the identifiable fish delivered to the Crescent Island tern colony (N = 590) were salmonids (this cumulative percent is an average of the previous weeks’ percentages). This compares to 73% juvenile salmonids in the diet of terns on Crescent Island by the same date in 2002.

Predator Activity

On May 12, a great horned owl was observed roosting on East Sand Island. Great horned owls are known predators of adult Caspian terns and have preyed on terns nesting on the East Sand Island colony in previous years. Owls, which hunt at night, can cause large disturbances to colonial nesting birds, sometimes leading to colony abandonment. In 1999, two great horned owls caused serious disruption to the East Sand Island tern colony and had to be removed to prevent the colony from abandoning. Thus far, we have no evidence to suggest that the great horned owl currently on East Sand Island has disturbed or preyed upon terns at the colony. If it is determined that the great horned owl is frequenting East Sand Island and preying on terns at the colony, efforts will be made to remove the owl from the island as soon as possible.

Disturbance rates at the East Sand Island tern colony were similar this week (0.7 disturbances/hr of observation) compared to the previous week. All of the disturbances to the East Sand Island tern colony this past week were caused by bald eagles. There were no disturbances observed at the Crescent Island tern colony this past week.

We observed predation on 2 tern eggs by gulls at the East Sand Island colony this past week, or 0.2 eggs depredated/hr of observation. We observed predation on 1 tern egg by gulls at the Crescent Island tern colony this past week, or 0.2 eggs depredated/hr of observation.

This past week, gull kleptoparasitism rates at East Sand Island were 16% of all known fates of fish delivered by terns, over twice the rate that was observed the previous week (7% of all known fish fates). At Crescent Island, gull kleptoparasitism rates this past week were 29% of all known fates of fish delivered by terns, nearly twice the rate that was observed the previous week (15% of all known fish fates). At the East Sand Island tern colony, kleptoparasitism is by glaucous-winged/western gulls, while at Crescent Island kleptoparasitism is by the smaller California gull.

Media & Other Visitors

There were no visitors to the East Sand Island tern colony site this past week.

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